Gas Controls, Valves, Regulators and Pressure Systems
This domain tests pressure management from the meter or tank to the burner: regulator types and performance, venting, overpressure protection, valve selection and appliance-level control components.
Regulators and pressures
- Natural gas is typically delivered at about 7 in. w.c. (1.75 kPa), with a manifold pressure near 3.5 in. w.c. Propane manifold pressure is typically 10–11 in. w.c.
- Two-stage propane systems: a first-stage regulator reduces tank pressure to roughly 10 psig, and a second-stage regulator delivers about 11 in. w.c. to the building.
- Regulator performance terms: lock-up (pressure rise at no flow), droop, spring range and orifice size. Adjust only within the spring range; a regulator that will not hold lock-up needs service, not more adjustment.
- Overpressure protection means a regulator plus a relief valve, or a monitor regulator. Propane liquid sections need hydrostatic relief; tank relief is set per B149.2.
Venting
Where a regulator is indoors, its vent must be piped outdoors at full size (never smaller than the vent connection), without traps or low points, terminating with a screened, downward-facing fitting, at least 3 ft from building openings and above snow or flood level. A blocked vent can overpressure the downstream system.
Valves and appliance controls
Listed manual shutoff valves, accessible and marked, at each appliance; quarter-turn ball valves with position indication are typical. Commercial trains use safety shutoff valves with proof of closure, high and low gas pressure switches and a vent valve. Never substitute a manually operated valve for a safety shutoff, and never use a valve to regulate pressure.
Orifices and capacity
Appliance input is set by orifice size and manifold pressure. Propane needs a smaller orifice than natural gas for the same input, and an appliance must never be re-orificed without the rating plate or a conversion kit.
Worked example
A rooftop unit's indoor appliance regulator is icing at its vent, and the burners soot. Diagnosis: the vent line is blocked by ice or condensate, so downstream pressure rises and combustion goes rich. The regulator itself is likely sound. Fix: reroute the vent outdoors in full-size pipe with no low points and a downward screen, then reset manifold pressure with a manometer while the burner is firing.
Traps that cost marks
Venting a regulator indoors or with undersized tubing; adjusting a regulator to compensate for low supply pressure; using a relief valve as a regulator; assuming natural gas and propane orifices interchange; and setting manifold pressure with the burner off.